1. Field of the Invention
The present invention relates to an optical semiconductor hermetic sealing package for accommodating an optical semiconductor element in its interior and an optical semiconductor module and an optical fiber amplifier using this optical semiconductor hermetic sealing package.
2. Description of the Related Art
In semiconductor devices for opto-electronics that need to operate at high speed for, for example, optical communication, in particular semiconductor modules such as those of light sources for exciting optical fiber amplifiers or of optical semiconductor amplifiers, hermetic sealing packages are employed for accommodating the optical semiconductor elements, driver ICs, etc., in their interiors.
As shown in FIG. 1 and FIG. 2, in a conventional optical semiconductor hermetic sealing package, typically, a bottom plate 2 made of Fexe2x80x94Nixe2x80x94Co alloy or Fexe2x80x94Ni alloy such as 42 alloy, or composite metallic material such as CuW is fixed to a frame made of metal such as Fexe2x80x94Nixe2x80x94Co alloy such as Kovar. In particular, a CuW bottom plate 2 is employed in an optical semiconductor hermetic sealing package where there is high power consumption and which is required to have heat-radiating capability.
A frame 1 comprising the side walls of the optical semiconductor hermetic sealing package is made by machining or injection molding of the above-mentioned Kovar etc., and usually comprises ceramic terminals 3 which are made of a plurality of ceramic sheets metallized at the required parts, and the lead terminals 4 made of Kovar. In some cases a construction is adopted in which part of the frame 1 is constituted of ceramic, which is an insulator, and is integrated with the ceramic terminals 3, or a construction is adopted in which the lead terminals 4 are inserted into through-holes provided in the frame 1, and fixed by sealing with glass.
In addition, a light transmitting window 5 is formed in the frame 1 to transmit light in the interior and exterior of the package. The light transmitting window 5 normally comprises a pipe of for example Kovar which is provided with a window member made of glass or the like to provide hermetic sealing. In some optical semiconductor hermetic sealing packages, any window material of glass or the like is not employed but the optical fiber is passed through the frame 1, the optical fiber being hermetically sealed to the transmitting window by solder brazing. In this case, only the window frame pipe is joined to the frame 1 of the package.
These components such as the frame 1, bottom plate 2 and lead terminals 4 are assembled into an optical semiconductor hermetic sealing package by joining by means of silver brazing or other solder brazing. In order to perform hermetic sealing with a lid, in order to prevent corrosion of the package, and in order to facilitate soldering during assembly of the semiconductor module, usually this entire optical semiconductor hermetic sealing package is subsequently subjected to gold plating. After packaging the optical semiconductor element etc within this optical semiconductor hermetic sealing package, finally, the lid (not shown) is air-tightly fixed to the upper end face of the frame 1 of the package by welding or solder brazing through a ring made of Kovar and so forth.
In the semiconductor module, an optical semiconductor element etc. is packaged in the interior of the optical semiconductor hermetic sealing package mentioned above. Specifically, as shown in FIG. 3, in the interior of the optical semiconductor hermetic sealing package, apart from an optical semiconductor element such as a laser diode (LD) element 6 and/or a photodiode (PD) element, a driver IC to drive this, a chip thermistor for temperature detection, and so forth are packaged by being mounted on a circuit board 7.
Optical semiconductor elements, in particular LD elements, are subject to inconveniences such as that, in addition to change of oscillation wavelength with temperature, their optical output falls at high temperature, with an extremely shortened life and adverse effect on their reliability. An electronic cooling device is therefore employed to control temperatures and cool the LD element and so forth. As shown in FIG. 3, the electronic cooling device has a construction in which a plurality of electronic cooling elements (Peltier elements) 8 constituted by crystals or a sintered body of a compound semiconductor BiTe are gripped between two insulating substrates 9 made of a ceramic sheet with metallized electrodes and wiring. Typically alumina or aluminum nitride is employed as the insulating substrates 9 of the electronic cooling device; in particular, aluminum nitride (AlN), which has good thermal conductivity, is employed when high heat radiating ability is required or when power consumption of the electronic cooling device is to be kept low.
The respective electronic cooling elements 8 are electrically connected by the metallized wiring on insulating substrates 9 and are provided with a pair of leads for electrical coupling between these and the semiconductor hermetic sealing package. In assembly of the optical semiconductor module, this electronic cooling device is soldered to the bottom plate 2 of the semiconductor hermetic sealing package and then a circuit board 7, on which have been pre-mounted optical semiconductor elements, such as an LD element 6 or PD element or other components, are mounted by means of solder brazing on one of the insulating substrates 9 of the electronic cooling device.
In an optical semiconductor hermetic sealing package which are constituted by parts made of different materials, as described above, after being formed into an optical semiconductor module, there was sometimes deterioration of performance of the electronic cooling elements such as BiTe elements in an environment endurance test at xe2x88x9240xc2x0 C. to +125xc2x0 C. as specified in MIL-STD. The deterioration of performance of such electronic cooling elements is caused by cracking resulting from thermal stress concentration in the electronic cooling elements such as BiTe elements, whose Young""s modulus is low and which is comparatively soft, due to warping of the bottom plate of the optical semiconductor hermetic sealing package and/or the circuit board etc on which the LD element or PD element within the semiconductor module has been mounted. The warping occurs due to the differences of thermal expansion coefficient between the members constituting the module.
When such deterioration of the performance of the electronic cooling elements occurs, the problem is experienced that power consumption becomes large because of a deterioration of cooling efficiency of the electronic cooling device, resulting, in the worst case, in temperature control of the optical semiconductor module becoming impossible to achieve because of its own generation of heat. Also, warping of the bottom plate of the semiconductor hermetic sealing package tends to occur the deviation of the optical axis of the optical system that effects optical coupling between the optical semiconductor element such as the LD element or PD element and the optical fiber, even if it does not give rise to any defects of the electronic cooling device. Such optical axis deviation causes the problem of lowering of output at the optical fiber terminals of the optical semiconductor module.
To deal with such problems, for example Japanese Patent Application Laid-open No. 6-314747 discloses the countermeasure of mitigating and absorbing warping by making the flange of the bottom plate thin, by grinding. Also, in Japanese Patent Application Laid-open No. 6-82659, the same effect is obtained by, instead of making the flange of the bottom plate thin, constructing only the flange of another metal having a small modulus of longitudinal elasticity. However, in both methods, processing is difficult and it was difficult to obtain sufficient strength.
Japanese Patent Application Laid-open No. 5-67844 proposes an optical semiconductor module in which the bottom plate of the package is also used as the insulating substrate of the electronic cooling device. Specifically, this is a method in which the bottom plate of the semiconductor hermetic sealing package is constituted by ceramic such as AlN. In this way, since no further insulating substrate is needed on one side of the electronic cooling device, miniaturization, in particular reduction in layer thickness, of the semiconductor module is achieved. However, with this method, there was the problem that, in addition to the difficulty of processing the holes etc. in the bottom plate which are provided for purposes of screw-fixing, the periphery of the holes of the bottom plate became liable to cracking when screw-fixing. Further problems included the fact that even after screw-fixing of the semiconductor module and heat radiating plate, the bottom plate was liable to cracking when warping of the heat radiating plate occurred.
Accordingly, in order to solve these problems, the inventors of the present application proposed a novel optical semiconductor hermetic sealing package in Japanese Patent Application Laid-open No. 11-074394 (Japanese Patent No. 3047864). This optical semiconductor hermetic sealing package comprises a frame made of metal, insulator, or a composite of metal and insulator, a first bottom plate made of metal fixed to this frame, and a second bottom plate fixed to the surface of this first bottom plate on the opposite side to the frame and of Young""s modulus larger than that of this first bottom plate. However, with this method, the flange of the first bottom plate must be subjected to bending processing in order to mount it on the heat radiating plate, and radiation of heat from the first bottom plate to the heat radiating plate can scarcely be expected.
In view of the above-mentioned prior art, an object of the present invention is to provide an optical semiconductor hermetic sealing package wherein warping of the bottom plate can be alleviated and which has an even more excellent heat radiating characteristic, as well as an optical semiconductor module and an optical fiber amplifier wherein, thanks to the use of this optical semiconductor hermetic sealing package, deterioration of the electronic cooling elements and/or optical axis deviation do not occur.
In order to achieve the above object, a hermetic sealing package for accommodating an optical semiconductor element provided by the present invention comprises: a frame made of a metal, an insulator, or a composite of metal and insulator, a first bottom plate made of a metal fixed to the frame, a second bottom plate whose Young""s modulus is larger than that of the first bottom plate and which is fixed to the surface of the first bottom plate on the opposite side to the frame, and a metal block plate that is fixed to the surface of flanges projecting on both sides of the first bottom plate on the side of the second bottom plate.
In the above-mentioned optical semiconductor hermetic sealing package according to the invention, a difference in level of 0 to 0.3 mm may be provided between the surface of the block plate fixed to the flanges of the first bottom plate and the surface of the second bottom plate. Also, the block plate may have a U-like shape and a gap for welding of an optical transparent window may be formed between the block plate, which is fixed to the flanges of the first bottom plate, the first bottom plate, the second bottom plate and the frame.
Also, in the optical semiconductor hermetic sealing package according to the invention, the Young""s modulus of the first bottom plate and the block plate may be 15xc3x97103 kg/mm2 or less and the Young""s modulus of the second bottom plate may be 25xc3x97103 kg/mm2 or more. Specifically, the second bottom plate may be made of a ceramic containing at least 90% of aluminum nitride or silicon carbide and having a flexural strength of at least 25 kg/mm2. Also, the first bottom plate and the block plate may be made of copper, Fexe2x80x94Nixe2x80x94Co alloy, or Fexe2x80x94Ni alloy.
In the optical semiconductor module and the optical fiber amplifier provided by the present invention, at least one optical semiconductor element mounted on a circuit board is accommodated in the interior of the optical semiconductor hermetic sealing package according to the invention as above. The optical semiconductor module and the optical fiber amplifier may comprise an electronic cooling device comprising Peltier elements gripped by a pair of insulating substrates between the first bottom plate of the package and the circuit board. In the optical fiber amplifier, an optical fiber is hermetically introduced into the optical semiconductor module and as the optical fiber an Er3+ doped fiber is preferably used to efficiently amplify the signal light.